1.6.6 Limitations and Artifacts of 2D Imaging
The potential of diagnostic ultrasound to display structures and tissue is influenced by a number of factors related to the way ultrasound waves behave in tissue. Not all patients have ideal imaging conditions. It may be necessary to find image windows that permit visualization of the heart. In addition, ultrasound waves undergo attenuation and therefore have a limited depth of penetration. Most importantly, ultrasound creates artifacts which may significantly alter image quality and falsely display structures or tissue.
Limitations of 2D Imaging:
Attenuation is the gradual loss of the intensity of a signal as it passes through a medium. The loss is linearly proportional to the imaging frequency. The higher the frequency, the higher is the attenuation. However, attenuation also depends on the tissues through which the wave travels. Some tissues such as bone (6-9 α(dB/(MHz.cm)) or dense connective tissue (1-2 α(dB/(MHz.cm)) have a high attenuation coefficient (impedance) and are difficult to penetrate while other tissues have a low coefficient (i.e. liver 0.5, blood 0.2 α(dB/(MHz.cm)). To compensate for attenuation, ultrasound systems employ a function known as time gain compensation. This function boosts signals from deeper tissues. All ultrasound scanners have time gain controllers which permit adjustment of receive gain at individual depths. Some scanners even have an automatic function that adjusts the image so that the display is more homogeneous.
α(dB/(MHz * cm))
Decrease in amplitude and intensity as an ultrasound wave travels through a medium
Factors that cause attenuation:
- Absorption (proportional to frequency)
- Transfer of energy from the beam to the tissue
Attenuation may cause artifacts such as shadowing and pseudoenhancement. This occurs because different regions of the ultrasound image show different degrees of attenuation. For example, when the ultrasound wave passes through very dense issue where marked attenuation occurs, there will be a loss of intensity (shadowing) behind the tissue (Figure a). Shadowing is a very common phenomenon in echocardiography. It occurs in the presence of intracardiac calcifications, behind prosthetic material (artificial heart valves) and bone. Conversely, when the ultrasound passes through tissue with a low attenuation coefficient compared to neighboring tissue, you will observe pseudoenhancement. The echoes behind will appear brighter than the neighboring tissue (Figure b).
Artifacts occur when ultrasound reflections do not correctly display actual structures or when they display structures that are not actually present. Artifacts can make it difficult to image, lead to misinterpretation and false measurements. Various types of artifacts may be encountered.
Acoustic shadowing happens when the ultrasound wave hits tissue with a large attenuation coefficient such as dense fibrous tissue, bone, or prosthetic material. The ultrasound waves cannot pass through and, consequently, there is less intensity behind the reflector (shadow) (Figure a). Such artifacts are a common problem in echocardiography.
Near field clutter
Near field clutter occurs due to high amplitude oscillations of piezoelectric crystals. It involves the near field and may hinder identification of structures that are close to the transducer - such as the apex - on a 4-chamber view (i.e. thrombus).
Reverberations occur when echoes are reflected several times from a surface. For example, when a wave hits the posterior wall it is reflected but the reflection does not entirely reach the transducer. It is reflected back from another surface, again towards the anterior wall.
This reflector is quite often the transducer itself. Thus there will be two signals from the posterior wall which will be displayed "twice" on the image.
The mirror artifact is similar to the reverberation artifact. Mirror image artifacts (mirroring) may occur in the presence of strong reflectors (i.e. pericardium) when the wave is reflected several times. Some of these waves have to traverse a longer path back to the transducer because they are reflected off at an angle to another interface which serves as a "mirror". As this wave takes longer to return, it is displayed in the far field distant to the true interface.
Side lobe artifacts
Sometimes ultrasound waves not only propagate along the axis of the beam but also radiate towards the side.
These "side lobes" may also cause artifacts, which appear as lateral blurring of the actual reflector.
Beam width artifact
When the ultrasound beam is wider than the diameter of the reflector being scanned, normal tissues that lie immediately adjacent to the lesion are included in the beam width, and their echo texture is averaged to that of the lesion. Thus, what appears to be the echogenicity of the reflector is really that of the reflector plus the averaged normal tissue. Owing to volume averaging, cystic lesions may falsely appear to be solid. Some subtle solid lesions may become impossible to distinguish from surrounding normal tissue and thus escape detection.
Artifacts may occur when image quality is poor OR good (such as mirror artifacts).
Tips to Avoid Artifacts:
- Know the pitfalls
- Know the anatomy
- Beware of strong reflections
- Use multiple views
- Artifacts are inconsistent.